The invention relates to a method and a device for monitoring the wear condition of a tool during machining of a metal workpiece by means of a machine tool.
Said wear monitoring process aims at deducing the wear progress of the tool from the sensor signal detected during the machining process. For this purpose sensors are installed in the working space of the machine tools such that the wear condition of the tool can be deduced from the detected sensor signal.
From JP 9323239 A a wear detector for lathe tools and a monitoring process are known, wherein the torsion moment acting upon the tool is measured and the measuring signals are supplied to a wavelet converter. By transforming the measuring signals from the sensor into wavelet coefficients with the aid of a wavelet function parameters can be determined which are representative of the wear condition.
It is an object of the present invention to improve the aforementioned process and the aforementioned device such that a quantitative and qualitative analysis of the wear condition with improved reproducibility is possible.
The present invention advantageously provides for reception by at least one vibration sensor of the vibrations, which are generated during the machining process, of a vibration system comprising at least the tool and the metal workpiece. The analysis of the vibration signal is more sensitive by approximately a factor of 50 with regard to the wear condition of the tool thus allowing a statement, which is exacter in terms of quantity and quality, on the wear condition of the tool. The determined wear condition parameters allow an evaluation of the wear progress at differerent locations of the tool. This is particularly important in the case of difficult-to-machine workpieces where it is essential that the workpiece is machined without any exchange of tool taking place. In this manner it can be decided prior to commencement of machining of a new workpiece whether a tool needs to be exchanged. The vibration analysis is superior to a large extent to the measurement of a torque or force signal since the changes in the force or torque signals do not necessarily allow conclusions to be drawn with regard of the wear condition, and said changes may be due to other reasons.
The vibration signal may be detected in the three-dimensional space in at least one space axis.
According to a partcularly preferrred aspect the vibration signal is detected in all three space axes. The wear condition parameters detected along the three space axes allow detection of the wear locations along the area of contact between the tool and the workpiece and/or along the cutting edge of the tool. In this manner a qualitative evaluation of the wear condition is possible and it can, e.g., be determined whether the tool is still usable for certain applications.
The three-dimensional vibration signal can be individually evaluated in each space axis, which results in a respective direction-oriented vibration signal. By means of the wavelet function respective direction-oriented wear condition parameters can be determined from the respective wavelet coefficient.
The three-dimensional vibration signal can be individually evaluated in each space axis, wherein after transformation of the three direction-oriented vibration signals a single resultant wear condition parameter can be determined from the wavelet coefficients in each space axis by means of the wavelet function, e.g. by addition, ratio formation, multiplication or vector addition.
According to another alternative the three-dimensional vibration signal can be individually evaluated in each space axis, wherein after transformation of the three direction-oriented vibration signals three wear condition parameters are determined from the respective wavelet coefficient by means of the wavelet function, the three wear condition parameters being combined, e.g. by addition, ratio formation or multiplication, to form a single wear condition parameter.
Alternatively, the vibration signal can be detected on the workpiece, the workpiece receiving means or the machine tool components directly or indirectly supporting the workpiece receiving means.
An acceleration sensor, a vibration velocity sensor, a vibration path sensor or, in the higher-frequency range, a solid-borne sound sensor or a sound emission sensor may be used as a vibration sensor.
Preferably, the wear condition of a workpiece is monitored during machining with interrupted cut.
A particularly preferred application is the wear condition monitoring of a milling tool during free-form milling.
The monitoring process can be e.g. applied during finish milling using a spherical milling tool. Here, the tool is exposed to different contact conditions due to permanently varying contact surfaces.
In the case of repeated machining actions on similar workpieces the wear condition can be evaluated on the basis of the change in the wear condition parameter relative to an initial parameter or relative to a given limit parameter at comparable cutting, engagement and/or contact conditions between tool and workpiece on similar or different workpieces.
In the field of tool manufacture frequently only a single formed part is produced such that it is necessary to define the given measuring locations with comparable cutting, engagement and/or contact conditions on one and the same workpiece.
The given measuring locations with comparable cutting, engagement and/or contact conditions between tool and workpiece can be determined from data contained in an NC production program of the machine tool, for which purpose e.g. the path and position data can be used. These measuring locations can be determined by a software which analyzes the NC production program in a computer (postprocessor). Said software may e.g. supplement the NC production program by program steps at the determined comparable measuring locations, said program steps allowing identification of the comparable measuring locations. Thus the NC production program can control an evaluation unit which compares and evaluates the wear parameters at the given measuring locations.
The software for analyzing the NC production program may further be capable of classifying a plurality of different comparable measuring locations, said measuring locations comprising different contact zones between tool and workpiece along the cutting edge of the tool but nevertheless comprising similar or comparable cutting and engagement conditions.
The given measuring locations with comparable cutting, engagement and/or contact conditions between tool and workpiece can be determined as a function of one or a plurality of the following parameters, namely workpiece blank geometry, workpiece finished part geometry, intermediate cut surfaces, machining parameters and tool geometry.
According to a preferred aspect of the present invention it is provided that, as a function of the wear condition parameters, the wear condition of the tool is indicated in terms of quantity on a two-dimensional or three-dimensional representation of the tool along the cutting edge of the tool. The representation of the tool can e.g. be shown on a display unit of the tool machine and the wear zones can be marked on the tool contour. On the basis of the optically displayed qualitative and quantitative wear condition an operator can decide in time whether the tool needs to be exchanged.